The present invention relates to a semiconductor integrated circuit device and, more particularly, to a technique which is effective when applied, in a semiconductor integrated circuit device including circuits for processing signals from an intermediate-frequency band to a high-frequency band, to a protective technique for preventing the electrostatic breakdowns of circuit elements constructing the semiconductor integrated circuit device.
A wireless communication system (or a wireless communication mobile terminal device, as will be shortly called the xe2x80x9cterminal devicexe2x80x9d) such as a digital cellular system has its circuits of transmission line and reception line constructed to include many discrete ICs (semiconductor integrated circuit devices). A representative example of the terminal device, of which the transmission/reception line circuits are constructed of the discrete ICs, is shown in FIG. 9. The discrete ICs are serviced from semiconductor parts makers such as NEC or RF Micro Device.
On the other hand, the technique of integrating the transmission/reception units into one chip is described on pp. 17 to 20 of xe2x80x9cHitachi Reviewxe2x80x9d, Vol. 81, No. 10 (1999-10), issued by Hitachi Reviewer Co., Ltd.
On the other hand, the electrostatic breakdown protecting circuit of IC is disclosed in Japanese Patent Laid-Open No. 230266/1989, for example. In this Laid-Open, there is disclosed an electrostatic breakdown preventing circuit in which a plurality of diodes are connected in series between the terminal of an integrated circuit and a power supply line or a ground line.
In Japanese Patent Laid-Open No. 202583/1995, on the other hand, there is disclosed a CMOS protective circuit corresponding to the CMOS circuit in which a plurality of power supply voltages are mixed.
Here will be described the prior art with reference to the accompanying drawings. FIG. 9 is a schematic block diagram showing transmission/reception circuits or the like, which are packaged in the terminal device of the prior art. The ICs are individually made functionally discrete, and the portions, as individually enclosed by squares, are the discrete ICs.
In this block diagram, there are shown a transmission line and a reception line, which are connected with an antenna 1 through a duplexer 2. Both these transmission line and reception line are connected with the not-shown base band.
The reception line is constructed by connecting the antenna 1, a band-pass filter 3 packaged in the duplexer 2, a low-noise amplifier 4, a band-pass filter 5, a reception mixer 6, a band-pass filter 7, a variable-gain controlled amplifier 8 and a demodulator 9 sequentially in series. The demodulator 9 is connected with the not-shown base band.
The transmission line is constructed by connecting a modulator 11, a variable-gain controlled amplifier 12, a transmission mixer 13, a band-pass filter 14, a transmission preamplifier 15, a high-output amplifier 16, a band-pass filter 17 packaged in the duplexer 2, and the antenna 1 sequentially in series. The modulator 11 is connected with the not-shown base band. On the other hand, the demodulator 9 and the modulator 11 perform the frequency conversion in response to a station signal inputted from a VCO 18. The reception mixer 6 and the transmission mixer 13 also perform the frequency conversion in response to the station signal from a VCO 19.
The signals (in electric waves) 10, as received by the antenna 1, are sequentially processed by the individual circuits of the reception line and are sent to the base band. On the other hand, the signals, as sent from the base band, are sequentially processed by the individual circuits of the transmission line and are emitted as the electric waves 10 from the antenna 1.
The portions, as enclosed by the squares, are the discrete ICs, as has been described hereinbefore. On the other hand, the internal small squares are electrostatic breakdown protecting circuits 20 (as will be called the xe2x80x9cprotective circuitsxe2x80x9d).
In the construction of the prior art thus far described, it is estimated from the handling notices described in the IC catalogue of each semiconductor parts maker that the variable-gain controlled amplifiers 8 and 12, the demodulator 9 and the modulator 11, or the intermediate-frequency band ICs of several hundreds MHz are provided with the protective circuits 20 for preventing the circuit breakdowns, as might otherwise be caused by the electrostatic charges from several tens to several hundreds V, to enhance the high breakdown voltages.
On the other hand, the high-frequency band ICs in the vicinity of 1 GHz such as the low-noise amplifier 4, the reception mixer 6, the transmission mixer 13, the transmission preamplifier 15 or the high-output amplifier 16 are described for the user to consider the static electricities, and it is also estimated that no protective circuit is included.
As one example of the protective circuit, there is known a protective circuit which is disclosed in Japanese Patent Laid-Open No. 202583/1995, as described hereinbefore. FIG. 10 is a schematic diagram in which a portion is added for easier illustration to the diagram of the protective circuit presented by the Laid-Open.
In FIG. 10, numeral 41 designates a protective circuit. Letter V designates an input or output signal to an integrated circuit, and this signal is inputted via a signal line 44 to an internal circuit 45 or outputted from the internal circuit 45. The protective circuit 41 is made of a diode-connected NMOS transistor 42 and an NMOS transistor 43. Specifically, the transistor 42 is connected at its shorted gate and drain with the signal line 44 and is connected at its source with a power supply voltage Vcc. On the other hand, the transistor 43 is connected at its drain with the signal line 44 and at its shorted gate and source with the ground.
On the other hand, one example of the protective circuit having diodes connected at multiple stages is disclosed in Japanese Patent Laid-Open No. 230266/1989, as has been described hereinbefore. FIGS. 11 and 12 are schematic diagrams in which a portion is added to the diagrams of the diodes for forming the protective circuit and the electrostatic breakdown preventing circuit of that Laid-Open, so as to facilitate the description.
As shown in FIG. 11, two diodes 51 and 52 are connected in series at two stages in the forward direction between the signal line 53 and the ground line 55 of an internal circuit 54. A signal is transmitted via the signal line 53 to the internal circuit 54. FIG. 11 omits the protective circuit on the power supply side, the action of which is identical to that shown in FIG. 10.
By the two-stage construction, a Von voltage is raised to suppress the electric current to flow through the protective circuit. FIG. 12 shows a sectional structure of the protective circuit elements constructed to have a two-stage connection. Over one face of a P-type substrate 61 forming an integrated circuit, there is formed an N-type epitaxial layer 62. On the surface side of this N-type epitaxial layer 62, there are formed a plurality of (or two, as shown) P-type diffusion layers 63 for wells. In the surface layer portion of the P-type diffusion layer 63, on the other hand, there is formed an N-type diffusion layer 64. Therefore, the P-type diffusion layer 63 and the N-type diffusion layer 64 form PN junction diodes (51 and 52).
Between the individual P-type diffusion layers 63, on the other hand, there is formed a P-type insulating diffusion layer 65 as a channel stopper. This P-type isolating diffusion layer 65 so extends all over in the thickness direction of the N-type epitaxial layer 62 as to reach the P-type substrate 61 at is lower end.
Over the surface on one side of the P-type substrate 61, on the other hand, there is formed a silicon dioxide film 66. This silicon dioxide film 66 is partially removed at its portion confronting the P-type diffusion layer 63 and the N-type diffusion layer 64, to form contact windows. Over the silicon dioxide film 66 including those contact windows, moreover, there is formed an aluminum deposited layer 67 in a predetermined pattern.
As a result, the two diodes 51 and 52 are connected in series in the forward direction by the aluminum deposited layer 67, and one diode 51 is connected at its cathode electrode 68 with the signal line 53 of the internal circuit whereas the other diode 52 is connected at its anode electrode 69 with the ground line 55.
In accordance with the miniaturization of the wireless communication mobile terminal device of recent years, there has been desired and developed a one-chip IC in which the transmission/reception band modulator/demodulator circuits and the high-frequency amplifications are integrated to correspond to the low-noise amplifier 4, the reception mixer 6, the variable-gain controlled amplifier 8, the demodulator 9, the modulator 11, the variable-gain controlled amplifier 12, the transmission mixer 13 and the transmission preamplifier 15, as shown in FIG. 9. It is necessary to protect the entire chip against the high breakdown voltage.
However, it has been found out that the one-chip IC of the prior art incorporating the protective circuit has the following problems.
(1) The high-frequency circuit including the low-noise amplifier 4, the reception mixer 6, the transmission mixer 13 and the transmission preamplifier 15 is provided at its input/output circuits with a matching circuit having a capacity and an inductance. With the inductance of this matching circuit, for example, a high voltage is generated by the change in the voltage at the input/output portions of the integrated circuit. Moreover, the output terminals of the individual circuits are biased in the DC manner to the potential of the power supply voltage. As a result, in response to the input signal, a signal having a potential no less than the power supply voltage is outputted. If the circuit having a construction of transistors of one stage is applied as the protective circuit as in the example of the prior art, a bias condition for allowing the electric current to flow through that protective circuit is established to raise a problem that the signal is distorted.
(2) On the other hand, the attachment of the protective circuit of the prior art raises a problem that the parasitic capacity is so high as to deteriorate the gain. This makes it hard to apply the protective circuit of the prior art to the circuit of the high-frequency portion.
(3) In the protective circuit having the transistors of a construction of two stages of the prior art, on the other hand, the element structure is constructed of thyristors. If a high static electricity is applied, therefore, the IC actions cannot be made because an overcurrent continuously flows unless the power supply is broken.
The problems, as found out by our investigations to be caused by the protective circuit for the high-frequency circuit, will be described with reference to FIGS. 13A and 13B and the subsequent Figures. First of all, the deterioration of the linearity will be described in the case of a circuit output.
FIGS. 13A and 13B illustrate the static characteristics of the NMOS transistors 42 and 43 of FIG. 10 and the actions of the high-frequency signals. With the circuit construction shown in FIG. 10, in the action state in which the power supply of the IC and the ground are connected with the power supply line over the substrate, little electric current flows through the transistors of the protective circuit, but the desired signal voltage is applied to the internal circuit 45, when the signal voltage Vcc desired by the circuit is applied to the signal line 44.
Next, if a positive static electricity V1 higher than the voltage Von, at which the transistors are turned ON, + the power supply voltage Vcc are applied to the signal line 44 at the time of handling the IC on the substrate, a diode forward current I flows through the NMOS transistor 42, as shown in FIG. 13A, but no overcurrent flows through the internal circuit so that the internal circuit 45 is protected.
Likewise for a high negative static electricity, if an electrostatic voltage V2xe2x80x2 no higher than xe2x88x92Von is applied, as shown in FIG. 13B, a diode forward current Ixe2x80x2 flows through the transistor 43 to protect the internal circuit 45.
Here, the high-frequency circuit adopts the method the matching is made by connecting the source terminal directly with the outside of the IC and by using the capacity and the inductance so as to amplify the output drastically, the power supply voltage is applied to the IC output terminal.
The internal circuit 45 of FIG. 10 corresponds to the low-noise amplifier 4, the reception mixer 6, the transmission mixer 13 or the transmission preamplifier 15 of FIG. 9, for example. When the power supply voltage Vcc is applied to the signal line 44 to superpose a high-frequency signal, a high-frequency voltage 46 around the graph origin of FIG. 13A is applied to the transistor 42. If the high-frequency voltage amplitude is no more than Von, little high-frequency current flows through the transistor 42, and the current amplitude of the internal circuit is outputted. If the voltage amplitude becomes large over Von, however, a nonlinear high-frequency current 47 flows to the transistor 42 so that it is superposed on the output current amplitude of the internal circuit to distort the output signal.
Under this action condition, the input amplitude of the desired wave of the reception circuit is at Von or smaller so that the distortion raises no serious problem. If the interfering wave in the near band, as cannot be filtered out by the upstream circuit stage, is so large as to have a voltage amplitude at Von or higher, however, the interfering wave causes a distortion to deteriorate the S/N characteristics in the reception band. In the transmission circuit, on the other hand, the desired wave has a large output amplitude so that the S/N deterioration occurs likewise at the distortion by the desired wave.
Here will be described the case of the input point of the circuit. Where the circuit includes enhancement type transistors such as Si bipolar transistors having a bias voltage at about +1 V at their input points, the protective circuit is normally OFF even if the high-frequency voltage amplitude is inputted, so that no electric current flows through the protective transistors thereby not to distort the input signal. In the case of the depression type such as the GaAs FETs, however, when a negative voltage V3xe2x80x2 is applied to the signal line so that the high-frequency signal is superposed, as shown in FIG. 13B, a high-frequency voltage 48 around V3xe2x80x2 is applied to the transistor 43. If the high-frequency voltage is no higher than xe2x88x92Von, it hardly flows through the transistor 43 so that the signal is transmitted as it is to the internal circuit.
If the voltage exceeds the value of xe2x88x92Von to a large amplitude, however, a nonlinear high-frequency current 49 flows through the transistor 43 and is superposed over the input current amplitude of the internal circuit to distort the input signal.
If the input signal has a distortion, this distortion is amplified by the internal circuit so that the amplified distortion is superposed on the distortion to be caused in the intrinsic signal, thereby to deteriorate the linearity. Like the aforementioned time of the output signal, the S/N ratio is deteriorated. This is because the protective circuit for the transistors of one stage diode-connected are provided on the power supply side and on the ground side for the signal lines.
In order to solve this problem, there is a method in which the diode-connected transistors are connected in series to raise the voltage Von so that little electric current may flow at the desired voltage for the internal circuit to act, as in the aforementioned protective circuit having the two-stage construction of FIG. 11. Moreover, the series connection reduces the parasitic capacity and the deterioration in the frequency characteristics.
According to this structure, however, a portion, as designated by 70 in FIG. 12, is given the thyristor structure by inserting the P-type diffusion layer of the channel stopper. The action principle of the thyristor is shown in FIG. 14. The thyristor is in the turn-OFF state to allow little electric current to flow, when it is in the forward voltage state and at a low voltage. When the voltage rises high, the thyristor is turned ON to allow a large electric current to flow. Therefore, the protective circuit of the diode construction having the multistage connection may be turned ON with a high voltage of static electricity. In order to prevent this, the power supply has to be broken. This breakdown is difficult to realize for the transistor structure of the prior art.
In the construction where the electric current flows from the ground to the power supply, as shown in FIG. 10, no problem arises, when the power supply and the ground of the IC are connected with the wiring lines of the substrate so that the power is supplied. When the IC is charged while being handled for its assembling work, however, the protective circuit may not function. With the IC being in the floating state and with the ground being open, for example, if a plus (+) or minus (xe2x88x92) potential difference is established between the voltage Vcc and the signal line 44, the voltage Vcc on the plus side with respect to the minus side of the signal line becomes inverse so that no electric current flows through the protective circuit. In other words, the protective circuit is weak against the negative static electricity with respect to the voltage Vcc, and the internal circuit may be broken by the high negative static electricity.
An object of the invention is to provide a semiconductor integrated circuit device for wireless communications, which has an electrostatic breakdown protecting circuit capable of protecting an IC reliably against positive/negative static electricities.
Another object of the invention is to provide a semiconductor integrated circuit device for wireless communications which has a protective circuit capable of protecting a low-frequency circuit (including a circuit of an intermediate-frequency band) and a circuit of a high-frequency band against the electrostatic breakdown.
Still another object of the invention is to provide a semiconductor integrated circuit device for wireless communications, which is excellent against the electrostatic breakdown while preventing the deteriorations of the linearity and gain of the high-frequency circuit.
The aforementioned and other objects and novel features of the invention will become apparent from the following description to be made with reference to the accompanying drawings.
A representative of the invention to be disclosed herein will be briefly described in the following.
For the circuit (or the low-frequency circuit) of the intermediate-frequency, protective circuits including diode-connected transistors of one stage are individually disposed on power supply lines and the ground line. For the circuit of the high-frequency band of the IC, there are incorporated, as the protective circuit having a low parasitic capacity and little deterioration of signal characteristics, multistage protective circuits in which the voltage Von is not exceeded even when a signal at the power supply voltage or higher is applied, in accordance with the action point of the applied circuit, and multistage protective circuits which are not turned ON even when a signal exceeding a negative bias potential is applied. On the other hand, the protective circuits are constructed to protect the IC, irrespective of the polarity of the static electricities to be established while the IC is being handled. Specifically, the protective circuits include: a first protective circuit for allowing an electric current to flow from a power supply line to signal lines at a protection time against an electrostatic breakdown; a second protective circuit for allowing an electric current to flow from the signal lines to the ground line; a third protective circuit for allowing an electric current to flow from the signal lines to the power supply line; and a fourth protective circuit for allowing an electric current to flow from the ground line to the signal lines. Moreover, the diode-connected transistors of the multistage connection are given a structure in which the elements (i.e., the diode-connected transistors) are isolated by an insulator capable of the thyristor action.
According to the means thus far described, (a) in the semiconductor integrated circuit device, each internal circuit of the high-frequency band is provided at its input/output portions with the protective circuit of the multistage structure so that the internal circuit can be prevented from the electrostatic breakdown, as might otherwise be caused by the high positive/negative static electricities. On the other hand, each internal circuit of the low-frequency band is provided at its input/output portions with the protective circuit of one stage so that it can be prevented from the electrostatic breakdown, as might otherwise been caused by the positive/negative static electricities.
(b) The output terminal of each internal circuit 29 of the high-frequency band is biased to the power supply voltage so that a signal at an output voltage or higher is outputted in response to the input signal. Where the protective circuit is made of the circuit including transistors of one stage construction, as in the prior art, therefore, the bias condition for the electric current to flow through the protective circuit is raised to distort the signal. In the invention, on the contrary, this signal distortion can be suppressed because the protective circuit of the multistage structure is adopted.
(c) The transistors are formed in the semiconductor regions which are electrically insulated from one another, so that the formation of the thyristor, as might otherwise be formed by the two-stage construction of transistors of the prior art, can be prevented.
(d) The protective circuit, as disposed at the input/output portions of each internal circuit of the high-frequency band is constructed to include the diode-connected transistors of the multistage structure. Therefore, the parasitic capacity is lowered to reduce the deterioration of the linearity or the gain of the high-frequency circuit.